Add HTMX.jl/Cobweb.jl

[frankier]

See https://discourse.julialang.org/t/what-can-we-learn-from-fasthtml/117663/7 and https://github.com/joshday/HTMX.jl

I'm not sure whether these are appropriate for a package extension, but I think maybe that's not a bad thing. Perhaps the documentation can just point out that this alternative "tags-in-julia" approach should also work well with Oxygen.jl?